A known torque converter with a multi-disc type lockup clutch for transmitting power by appropriately engaging a pump impeller and a turbine runner with each other can cool down or radiate with a degree of certitude heat from multiple clutch plates provided in the lockup clutch. Such a torque converter is disclosed in U.S. Pat. No. 5,441,135 and Japanese Patent Application No. 2001-116110.
According to the torque converter with a lockup clutch disclosed in the U.S. Pat. No. 5,441,135, a support piece for supporting friction discs of a lockup clutch in an axial direction, and a working pressure chamber (piston chamber) for accommodating an axial piston of a clutch member are provided so that the support piece and the working pressure chamber are prevented from relatively moving to annular friction disc carriers. The friction discs are alternately and torsionally held on two annular friction disc carriers and subjected to a centripetal flow between a turbine wheel outlet and a torque converter return conduit.
Further, according to the torque converter with a lockup clutch disclosed in the Japanese Patent Application No. 2001-116110, a throttle passage structure is provided between a converter chamber and a torque converter oil drain passage. A lockup chamber has a closing structure between a space formed by a turbine runner and a torsion damper, and the torque converter oil drain passage, so that oil introduced from the converter chamber into the lockup chamber is discharged into the torque converter oil drain passage via the lockup clutch.
According to each of the aforementioned torque converters with a lockup clutch, operation oil is controlled at a predetermined pressure level at the source of oil pressure, and then guided into the converter chamber from an oil supply passage. Then, the operation oil flows into the lockup chamber by passing through a gap formed between a turbine assembly and a converter cover, and is delivered to the oil drain passage via the multi-disc lockup clutch. Operation oil generates heat when a rotational difference between the pump impeller and the turbine runner of the torque converter is large (i.e. a slip ratio is high) and then a temperature of operation oil is increased. Operation oil discharged from the oil drain passage is generally delivered to a lubricating system of each portion of an automatic transmission main body after being cooled down in a cooler, and then returned to an oil pan. Operation oil stored in the oil pan is pumped again so as to be controlled at a predetermined pressure level, and guided to the converter chamber. That is, operation oil is maintained at a substantially constant pressure level in the converter chamber. Heat generated in the torque converter is accordingly discharged to the outside via operation oil.
Since operation oil flowing through the gap formed between the turbine assembly and the converter cover circulates by passing through gaps formed in the clutch discs of the lockup clutch, in a case where the lockup clutch is in engagement state, no gaps are formed between the clutch discs. The flow of operation oil is prevented, and thus results in a reduction in the flow amount of operation oil. Meanwhile, a flow resistance in the oil drain passage is not changed and thus pressure is reduced in a void formed on a side of the lockup chamber relative to a piston for lockup, which is in the vicinity of the oil drain passage. The pressure of operation oil applied to the piston chamber is controlled separately from that applied to the converter chamber, and is thus not influenced by the temperature of operation oil in the converter chamber, or in the lockup chamber. Therefore, when the pressure of operation oil in a portion surrounded by the piston for lockup and the lockup clutch is changed, a force for engaging the clutch is changed even if the pressure of operation oil in the piston chamber is at a constant level.
Further, the pressure in the converter chamber is high when the temperature of operation oil is high while the pressure is low when the temperature of operation oil is low. In a case where the temperature of operation oil increases by the heat generated in the torque converter, viscous resistance of operation oil diminishes, the amount of discharge from the torque converter increases and as a result pressure in the converter chamber drops.
Thus, a need exists for a torque converter with a lockup clutch that can prevent a change of oil pressure in a portion surrounded by a piston for lockup and a lockup clutch.
Further, a need exists for a torque converter with a lockup clutch that can prevent pressure in a converter chamber from decreasing even when the temperature of oil becomes high. The present invention has been made in view of the above circumstances and provides a torque converter that meets both criteria.